The subject matter disclosed herein relates to a power conversion system for use with an alternate energy source, and more specifically, to a power conversion system including a first inverter for use in grid-tied operation and a second inverter for use in off-grid operation.
Historically, electrical power has been supplied to a consumer by a utility power grid. However, rising energy costs, environmental concerns, a desire or need for backup energy sources, or a combination thereof have led to an ever increasing base of consumers utilizing alternate energy sources, such as wind turbines or photovoltaic cells, to supply at least a portion of their electricity requirements.
These alternate energy sources are typically connected in parallel with the utility grid such that either energy source may supply energy to the consumer. The utility grid is connected via a bidirectional power meter to an input point at the load center, which distributes electrical energy to the various electrical loads. The alternate energy source is also connected to the load center via a grid-tied inverter. The grid-tied inverter monitors the utility grid and converts the electrical energy generated by the alternate energy source to a voltage in phase with the utility grid. If the energy generated by the alternate energy source exceeds the demands of the loads, then the excess energy is transferred to the utility grid. If the energy generated by the alternate energy source is less than the demands of the loads, then the difference in the amount of energy generated by the alternate energy source and the demand of the loads is supplied by the utility grid. Depending on the generating capacity of the alternate energy source and the consumer's load demands, the alternate energy source may reduce or eliminate energy bills from the utility provider or even cause the utility provider to buy excess generating capacity from the consumer.
In addition, the alternate energy source may be capable of generating energy during periods of power failure in the utility grid. Because the output of the grid-tied inverter and the utility grid are both connected to the input of the load center, the potential exists that electrical power generated from the alternate energy source may be supplied back to the utility grid during a power failure. However, utility crews working to restore the utility power disconnect portions of the grid and expect no power to be present beyond the point of disconnection. The presence of power from an alternate energy source creates a potentially hazardous environment in such circumstances. Consequently, electrical codes typically require that the grid-tied inverters be disabled during a power outage to avoid the potential of unexpected power being present on the utility grid. While disabling the grid-tie inverter prevents potentially hazardous energy transfer from the alternate energy source to the grid, it also prevents the alternate energy source from acting as a backup generator during the outage and reduces the potential energy savings that may be realized from the alternate energy source.
Thus, it would be desirable to provide a power conversion system that both prevents potentially hazardous energy transfer from the alternate energy source to the grid and allows the alternate energy source to provide energy to at least a portion of the consumer's electrical loads during a power outage.